Glucagon

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Chapter: Essential pharmacology : Insulin, Oral Hypoglycaemic Drugs and Glucagon

A hyperglycaemic principle was demonstrated to be present in the pancreatic islets just two years after the discovery of insulin in 1921. It was named ‘glucagon’. Glucagon is a single chain polypeptide containing 29 amino acids, MW 3500.


GLUCAGON

 

A hyperglycaemic principle was demonstrated to be present in the pancreatic islets just two years after the discovery of insulin in 1921. It was named ‘glucagon’. Glucagon is a single chain polypeptide containing 29 amino acids, MW 3500. Beef and pork glucagon are identical to human glucagon. It is secreted by the α cells of the islets of Langerhans.

 

Regulation Of Secretion

 

Like insulin, glucagon is also derived by cleavage of a larger peptide prohormone. Its secretion is regulated by glucose levels, other nutrients, paracrine hormones and nervous system. Glucose has opposite effects on insulin and glucagon release, i.e. high glucose level inhibits glucagon secretion and it is more sensitive to orally administered glucose: suggesting that the same gastrointestinal incretins which evoke insulin release may be inhibiting glucagon secretion. FFA and ketone bodies also inhibit glucagon release. Amino acids, however, induce both insulin and glucagon secretion. Insulin, amylin and somatostatin, elaborated by the neighbouring β and D cells, inhibit glucagon secretion. Sympathetic stimulation consistently and parasympathetic stimulation under certain conditions evokes glucagon release.

 

 

Actions

 

Glucagon is hyperglycaemic; most of its actions are opposite to that of insulin. Glucagon causes hyperglycaemia primarily by enhancing glycogenolysis and gluconeogenesis in liver; suppression of glucose utilization in muscle and fat contributes modestly. It is considered to be the hormone of fuel mobilization. Its secretion is increased during fasting: this serves to maintain energy supply by mobilizing stored fat and carbohydrate as well as by promoting gluconeogenesis in liver. It plays an essential role in the development of diabetic ketoacidosis. Increased secretion of glucagon has been shown to attend all forms of severe tissue injury.

 

Glucagon increases the force and rate of cardiac contraction and this is not antagonized by β blockers. It has a relaxant action on the gut and inhibits gastric acid production.

 

Mechanism Of Action

 

Glucagon, through its own receptor and coupling Gs protein activates adenylyl cyclase and increases cAMP in liver, fat cells, heart and other tissues; most of its actions are mediated through this cyclic nucleotide.

 

Glucagon is inactive orally; that released from pancreas is broken down in liver, kidney, plasma and other tissues. Its t½ is 3–6 min.

 

Uses

 

1. Hypoglycaemia due to insulin or oral hypoglycaemics; use of glucagon is secondary to that of glucose; only an expedient measure. It may not work if hepatic glycogen is already depleted: 0.5–1 mg i.v. or i.m.

 

2. Cardiogenic shock to stimulate the heart in β adrenergic blocker treated patients. However, action is not very marked.

 

3. Diagnosis of pheochromocytoma 1 mg i.v. causes release of catecholamines from the tumour and markedly raises BP. Phentolamine should be at hand to counter excessive rise in BP.

 

GLUCAGON 1 mg inj.

 

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